A brushless permanent magnet motor typically includes windings on the stator, rotating permanent magnets, and a position sensor for indicating the rotor position. The winding energization is generally controlled by solid state switches that are responsive to the position indications to energize the windings in the proper commutated sequence. Control of motor torque is achieved by controlling the magnitude of the winding excitation current.
U.S. Pat. No. 4,447,771 to Whited, entitled "Control System for Synchronous Brushless Motors" (the '771 patent), and U.S. Pat. No. 4,546,293 to Peterson et al., entitled, "Motor Control for a Brushless DC Motor", both describe a system in which both the phase and magnitude of a motor's winding excitation currents are controlled. A quadrature phase relationship normally exists between the rotor field and the rotating stator magnetic field. The phase angle is varied from the quadrature relationship to compensate for the fundamental current lag, which is a predetermined function of the motor speed. By dynamically varying the phase angle an improved motor performance is achieved over a wider speed range.
In addition, U.S. Pat. No. 4,651,068 to Meshikat-Razavi discloses a brushless motor drive circuit that utilizes current shaping in addition to a phase advancing technique as a function of motor velocity. The waveform compensation is especially effective in motors with trapezoidal magnetic field distribution.
In a manner similar to the '771 patent, U.S. Pat. No. 4,490,661 to Brown et al., entitled "Control System for Synchronous Brushless Motors Utilizing Torque Angle Control" (the '661 patent), employs "torque angle factors" to vary the phase relationship between the rotor field and the rotating stator magnetic field as a function of the motor load, in addition to the phase advance as a function of the speed.
This patent may be applicable to motors where the inductance value of the stator is a function of the rotor angle, typically notable when rotor magnets are buried inside the rotor. For these types of motors, U.S. Pat. No. 4,649,331 to Jahns teaches us how to operate the motor at a high speed, constant power range by automatically following the voltage limit ellipse.
Although the fundamental principles of phase advancing techniques are revealed in the previous patents, a method to obtain the optimal phase advance values is highly nonlinear and usually requires torque plot at various phase advance angles at each operating condition (speed and torque).
The typical calibration procedure for arriving at the torque angle factors used in determining the angle advance in the '771, '661 and other patents would use a test motor and drive and a dynamometer with torque and speed sensor. First, the motor is started to run with a load so that its temperature reaches its predetermined temperature. Once stable speed and temperature were achieved the angle advance is manually adjusted while observing the dynamometer reading to determine the angle that gives the maximum torque. The calibration procedure was repeated for the desired number of motor speeds and/or loads to create a table of torque angle factors. The calibration procedure would be repeated for each motor type, and in some cases, for each motor. This prior calibration method was costly and time consuming and provided torque angle values for only a finite number of motor speeds.
An object of the present invention is to provide a formula to calculate the amount of speed and load angle advances based on motor and drive parameters.
Another object of this invention is to provide a command boost technique to further increase the torque from the motor. Although many prior patents, as mentioned above, dealt with speed and load advance angles, they all exhibit magnitude reduction of the actual torque at high speeds. The magnitude boost can also be calculated from the motor parameters to improve high speed torque when operated with the torque mode.